The major objective of this study is to test for conformational identity or differences between dissolved and crystalline human erythrocyte carbonic anhydrase B. The experimental approach is to react crystalline and dissolved enzymes with radioactive iodoacetate and determine which histidines are modified in the two states. Reaction of dissolved enzyme with 200 mM sodium iodoacetate for 14 days at 22 degrees yielded a modified enzyme with 2.4 residues of tele-carboxymethyl histidine, 0.8 proscarboxymethyl histidine, and 1.1 dicarboxymethylhistidine. Thus, many of the 11 histidyl residues are unreactive. Proteolysis and purification of peptides containing modified histidines showed that histidine 200 is present only as the telecarboxymethyl derivative and that histidine 243 is present as a mixture of all three derivatives. These results are consistant with the structure of the crystalline enzyme. Reaction of 200 mM iodoacetate with crystalline enzyme at 22 degrees result in destruction of the crystal structure, but the crystals remain intact for at least 14 days if the reaction is done at 4. The initial course of reaction is similar to that with dissolved enzyme. Studies with several imidazole and barbiturate buffers show that low concentration of buffers activate the CO2-hydrating activity (up to 6-fold) whereas at higher concentrations, most of these same buffers are inhibitors. BIBLIOGRAPHIC REFERENCES: P.L. Whitney and H. Brandt (1976) Effects of two ionizing groups on the active site of human carbonic anhydrase B. J. Biol. Chem. 251, 3862-3867. P.L. Whitney (1977) Activation and Inhibition of CO -hydrating activity of human erythrocyte carbonic anhydrase B by buffers. Fed. Proc. 36, 762.